1. Technical Field
The present disclosure relates to cardiac MRI and, more specifically, to the automatic determination of a field of view in cardiac MRI.
2. Discussion of Related Art
Magnetic resonance imaging (MRI) is a medical imaging technique in which a human subject can be imaged in three-dimensions with a great deal of detail pertaining to the differentiation of different forms of bodily soft tissue. Thus MRI is well suited for the visualization and diagnosis of cardiovascular disease. In MRI, the human subject is exposed to a powerful magnetic field which aligns the nuclear magnetization of hydrogen atoms in water within bodily tissues. Radiofrequency fields are used to systematically alter the alignment of this magnetization and the hydrogen nuclei then produce a rotating magnetic field detectable by the scanner.
Structural image data may be generated from the received data signals to construct an image of the body. For example, the structural image may be generated from a number of spatial frequencies at different orientations. Frequency and phase encoding are used to measure the amplitudes of a range of spatial frequencies within the object being imaged. The number of phase-encoding steps performed may be selected to determine how much imaging data may be collected.
As MRI uses magnetic and radiofrequency fields to perform visualization, the patient is not exposed to potentially hazardous ionizing radiation as would be the case with CT scans.
In MRI, spatial resolution may generally be determined by the size of the field of view (FOV) and the number of phase-encoding steps performed during scanning. Thus, to achieve a greater spatial resolution and a higher level of image detail, the FOV may be reduced and/or the number of phase-encoding steps may be increased. For a given number of phase-encoding steps, a smaller FOV will result in a higher resolution MR image.
However, MR images may be prone to wrap-around artifacts in which a part of the imaged anatomy from the periphery of the FOV appears on an opposite side of the periphery of the FOV, with respect to the phase encoding direction, as if structures that should be on one side of the image appear on an opposite side of the image. Wrap-around artifacts may occur, for example, when the boundary of the FOV intersects with the subject's body. If the FOV is too small, the wrap-around region may intersect anatomy displayed on the opposite side of the image.
FIG. 6 is a set of four MR images (a), (b), (c), and (d) illustrating wrap-around artifacts. The four images show distinct MR views, however, in image (b), it can be seen that the right margin of the image 61 has been cut off and appears as wrap-around artifact 62 on the left margin. Similarly, in image (d), it can be seen that the bottom margin of the image 63 has been cut off and appears as a wrap-around artifact 64 on the top margin.
Wrap-around artifacts are not a problem for MR imaging so long as the region of the body that is the focus of the MR study is sufficiently far from the periphery of the FOV so that any wrap-around artifacts do not obstruct the region of the body that is the focus of the MR study. Accordingly, it is important that the FOV not be set too small or there will be an increased possibility that wrap-around artifacts will interfere with the diagnostic value of the study.
Accordingly, an optimal FOV may be selected such that the size of the FOV is small enough to produce a sufficiently high resolution image, and yet large enough in the phase encoding direction to prevent the occurrence of wrap-around artifacts from obstructing the region of the body that is the focus of the MR study, which may be, for example, the heart.
The FOV is accordingly manually selected by a trained medical practitioner or technician to achieve the desired results. However, this manual selection may be time consuming and prone to human error.